From now on, test_download will run these tests. That means we benefit not only from the networking setup in there, but also from the other tests (for example test_all_urls to find problems with _VALID_URLs).
This moves playlist tests where they belong, i.e. to the extractors themselves.
Additionally, all our network interaction configuration for tests in test_download now applies to playlist tests as well.
This is suboptimal, but at least this way we will need to look at the logs
only to check for network errors that happen too often, instead of
parsing a ton of lines each time to see if there is some true test failing